The invention relates to a ship with a prow.
In order to reduce the power required for the propulsion of ships and in order to save fuel, the various technical solutions have been proposed. Thus, it is known e.g. from German Pat. No. 1,207,820 to improve the flow to the propeller and consequently the propulsion efficiency by an asymmetrical construction of the stern. In the case of a hull for a single-propeller ship or a ship with a central propeller with a low Froude number and correspondingly high volumetric efficiency, the shape of the stern is made asymmetrical in such a way that that part of the stern above the propeller shaft is turned compared with that part of the stern located below the shaft counter to the rotation direction of the propeller and consequently in the vicinity of the propeller race, the axes of the horizontal sections through the hull are inclined away from the median longitudinal plane of the ship counter to the rotation direction of the propeller and namely so as to increase from that of the root of the propeller blades to the area facing the ends of the blades, so that viewed from aft, the propeller post forms with the median longitudinal plane above the propeller shaft an angle inclined counter to the rotation direction of the propeller.
This shape has mainly been developed for full-built ships in order to improve the flow against the propeller and in order to increase the stern fineness compared with symmetrical sterns, without any loss of propeller efficiency. Whilst retaining the original fineness or displacement of the stern, it is possible to obtain, as required, a speed increase or, for the same speed, a reduction of the propulsive output and consequently fuel consumption.
Output and/or fuel savings have been achieved in the case of various ships having such sterns. In the case of these ships, that part of the stern located above the propeller shaft has been turned counter to the rotation direction of the propeller compared with the part below the shaft and as a result of a conventional U-shaped frame below the propeller shaft, a configuration of the isotach of the wake in this area has been achieved, which is very close to the ideal case of rotational symmetry. These ships have coefficients of fineness between appoximately 0.75 and 0.83. In the case of relatively faster ships in the range of higher Froude numbers and lower volumetric efficiencies, it is known to favourably influence the course of the isotach of the wake by providing stern reinforcements.
The use of maximum propeller diameters with correspondingly low speeds is often sought for economic reasons, because this can lead to an efficiency improvement and to a reduction in the required power or fuel consumption. However, it has been found that in the case of extremely large propeller diameters compared with the draught, the wake conditions deteriorate and vibration-exciting forces and cavitation occur, so that as a function of the type of ship, the fineness and the speed, special additional technical means must be provided for obviating these disadvantages, which are disadvantageous from the economic standpoint.
In addition, a ship with ice-breaking characteristics is known, whose hull is provided with a pontoon-shaped prow part above the waterline with a front surface extending over the entire beam and which in its lower part slopes upwards and forwards in a pronounced manner and with approximately parallel sectional areas on the lower edges of the side walls. The part of the underwater prow following onto the pontoon-shaped prow part is V-shaped and with laterally upwardly and forwardly inclined transition surfaces abutting with a forwardly inclined stem passes into the pontoon-shaped prow part (German Pat. No. 2,343,719).
If a ship constructed in this way passes through a layer of ice, it has been found that in the latter a channel with smooth straight edges is formed and the width of this channel corresponds to the width of the ice breaker. However, during the ice breaking process, undesired, disadvantageous phenomena can occur. Thus, in the case of a ship according to German Pat. No. 2,343,719 the parallel prow sides above the cutting edges lead to a frictional force being exerted but the cut ice surface on the outer plating of the ship, causing a not inconsiderable proportion of the resistance of the latter in the ice. Furthermore, when the hull is listing, this frictional force can be increased through the waterline width of the listing ship being increased compared with the cut channel in the ice, so that a squeezing or jamming effect is produced.
This squeezing effect is increased if the layer of ice is subject to horizintal stresses at right angles to the direction of travel, so that ice pressure on the sides is increased. This leads to an effect of forces according to FIG. 2, in which the pressure direction is X, the starboard side of the cross-sectionally diagrammatically indicated hull is SB, the port side is BB, the layer of ice with horizontal compressive force is E1 and the layer of ice without compressive force is E, which leads to a bending process in the ice layer/ice breaker/ice layer system, considered in the action line at right angles to the direction of movement. On the port side, this leads to a raising of the ice layer and ice breaker, but to a lowering on the starboard side. As in any bending process, there can be an interchange between the rising and lowering motions. The greater these pressures, the greater the list and friction.
According to German Pat. No. 2,530,103, an ice breaker with a pontoon-shaped prow part above the waterline is constructed in such a way that said prow part has sliding and breaking profiles on the lower edges of its two side walls, which are parallel to one another and extend up to the V-shaped part of the underwater prow. The facing wall faces of these profiles slope upwards and carry the cutting edges.
Such an ice breaker with planar, upwardly sloping faces of its pontoon-shaped prow slides on the ice to be broken without the central part of the prow coming into contact with the ice. As a result of the forces exerted on the ice by the two cutting edges, a single-part ice floe is broken and essentially has the same width as the prow. Finally, at the sharply upwardly directed underwater stem, which essentially forms an extended keel in the vicinity of the V-shaped underwater prow, this ice floe due to its buoyancy passes into an unstable state of equilibrium, from which the floe is tilted to one side and floats laterally under the fixed ice layer, to give an ice-free channel.
It has been found that in the case of such an ice breaker, improvements are necessary in connection with the reliable guidance of this ship when sliding onto the fixed ice layer and in connection with the following shear fracture behaviour with highly fissured ice surfaces, such as pack ice ridges which have frozen onto one another, and under different and optionally varying ice conditions, such as strength, thickness, etc. A ship according to German Pat. Nos. 2,343,719 and 2,530,103 has the disadvantage that the broken ice floes with the complete ship's beam can float in uncontrolled manner to any side, but often do not completely disappear beneath the unbroken ice layer or are broken into a larger or smaller number of fragments by uncontrollable crushing operations, which can also not be passed laterally under the fixed ice layer. When travelling in ice-free areas, the ship is subject to considerable, damaging buffeting by the sea.
DOS No. 2,112,334 discloses an ice breaker, whose hull passes into an underwater prow with two wedge-shaped ice breaker stems forming between them a channel. At the rear end of the channel, a snowplough-like guidance means is arranged beneath the bottom of the ship. The resulting large number of small ice floes cannot be passed under the lateral, fixed ice layer and instead float in the gap between the hull and the lateral fixed ice layer, causing increased friction on the outer plating of the ship, or they collect in the channel and slide midships under the ship into the propeller area. Thus, such a ship has increased power requirements and the propellers are exposed to the harmful action of ice floes.
It is also known that ships, particularly ice breakers, with pontoon-like prow part when travelling in ice-free, open water are subject to severe buffeting by the waves striking the bow, leading to considerable vibrations on the hull, which makes it difficult to maintain course and increased power is needed for propulsion purposes.